I agree with Kahtar's point that, even conceding possession of a man's body by a fallen angel, the human sperm would presumably still be human--leaving unanswered, the question as to why their offspring would be so unusual.
With regard to angels "possessing" humans: I've pondered that in the past, but it seems to me, there is a distinction in the Bible between fallen angels and demons. In both the Hebrew and Greek texts, there is a marked difference in the concepts behind their root words. And, we still have the question, "if the fallen angels were consigned to the abyss, we have to wonder how they were still free to roam about and inhabit the bodies of people in Jesus' day?"
There is a traditional belief among some Bible scholars that demons are the disembodied spirits of fleshly beings who inhabited the earth prior to the creation of man--perhaps having died in a deluge similar to that in the time of Noah. This, the theory goes, explains why the earth was covered in water prior to its restoration, as described in Genesis 1).
We must tread lightly when speculating about Biblical matters, especially when lacking sufficient corroboration. Yet, the idea of evil spirits taking control of living bodies, lends merit to the idea that they once had one had one of their own. (I know I'm digressing, but this matter does come to bear on the study). This question is wide open, but what Scriptural evidence do we have that suggests angelic possession, in the sense of demonic possession? Or is there evidence to support the idea that the two spirit beings are one and the same?